Many electronic display devices that we are familiar with are emissive, for example laptop screens, desktop monitors and televisions. Such devices, whilst highly visible indoors, are hard to see in bright (particularly outdoor) conditions. Additionally, such devices are often power-hungry due to the need to generate light either via a backlight in the case of liquid crystal displays (LCDs) or within the display itself in the case of cathode ray tubes (CRTs) or organic light emitting displays (OLEDS).
An alternative to such emissive displays is to use a reflective display, in which the image is generated by modulating the intensity of the reflected ambient light. Such displays have the advantage of working with (rather than competing against) the strength of the ambient light, and hence are a good solution for displays which are used primarily in bright conditions. They also tend to have much lower power consumption, because there is no light generation involved. If necessary, the display can also be fitted with a front-light (which provides illumination from the front surface of the display) so that it can be read in darker environments also. LCDs are notoriously inefficient when used in either transmissive or reflective mode, due to the losses involved in the polarisers, colour filters and black mask in the display structure. When used in transmission, a high brightness display can nonetheless be achieved by using a bright backlight, but this comes at the cost of high power consumption. When used in reflective mode, however, these inefficiencies result in a low reflectivity and as a result poor image brightness, much less than the brightness that would be achieved from the ultimate reflective display: paper.
In recent years, monochrome alternatives to reflective LCDs have emerged onto the market-place, the most commonplace being E-Ink technology. This has been used primarily to make monochrome e-book readers, such as the Amazon Kindle®. The white state reflectivity of such e-books is around 35-40% which, whilst comparable with newspaper, still falls short of the reflectivity from a quality white piece of paper. A more recent emerging technology is the Mirasol® display from Qualcomm MEMS Technologies, which uses an interference-based MEMS method to generate a switch between pixels which appear either black or green. Qualcomm claim 45% reflectivity in these displays which they term “bichrome” because the bright state is green rather than white.
These emerging technologies, whilst perhaps beginning to out-perform monochrome active-matrix LCDs, are still not reflective enough to be able to generate a high reflectivity colour image. To create a colour image, E-Ink would simply need to add colour filters to their display, which would cut the white state reflectivity down by about ⅓ to around 11-13%. Qualcomm claim to have a colour version of their Mirasol® display which works not by having colour filters as such, but nonetheless by having coloured sub-pixels which (in their bright state) reflect either red, green or blue. Their projected white state reflectivity for such displays is 25%, which would be the highest reflectivity colour display on the market today.
However, whilst this performance is impressive, it still falls rather short of the white state reflectivity we are accustomed to in printed colour images on paper (˜60-70%). One technology which has the potential to achieve this target is that of electrowetting.
The term electrowetting refers to the voltage dependent wetting of a droplet of fluid on a surface, and can be used to manipulate small droplets of fluid. It has been applied to make variable focus lenses by Varioptic, and is also being developed for “lab-on-a-chip” applications that involve moving very small quantities of biological fluid around in a single plane. It can also be used for making displays, and there is a plethora of different ways in which this can be realised.
Perhaps the simplest way of creating an electro-wetting display is illustrated in FIGS. 1(a) and 1(b). A lower substrate 1a has a series of electrodes 2 which could be either transparent or reflective, according to whether the display is intended to be transmissive or reflective. On-top of the electrodes 2 is a transparent dielectric insulating layer 3 and a transparent hydrophobic layer 4. Spaced apart, and positioned above and parallel to the lower substrate 1a is an upper substrate 1b, with a uniform transparent electrode 5 and another hydrophobic layer 4. The two substrates sandwich a fluid layer which includes two different types of fluid which are immiscible with each other: these are the electrowetting fluids 6 and 7. One of these is fluid 6, a non-polar fluid such an oil (e.g. dodecane). The other is fluid 7, a polar fluid such as a weak solution of ions (e.g. KCl) in water or a mixture of water and ethyl-alcohol. The fluids are dispensed so that the polar fluid 7 consists of droplets that are substantially equal or slightly greater in area to the size of the electrodes 2, and positioned at every other electrode 2, as shown in FIG. 1(a). One of the fluids is transparent, and the other is coloured with a dye, often black. In this illustrative example, the polar fluid 7 will be black and the non-polar fluid 6 will be transparent. In the case of the example illustrated in FIGS. 1(a) and 1(b), each pixel 21 includes two electrodes 2a and 2b and the pixels 21 are arranged in an array of rows and columns as is conventional. Each pixel 21 also includes a black mask 8 which obscures some of the pixel 21 from a viewer 9, in this case, at least the part of the pixel 21 that is defined by electrode 2a. When a voltage is applied between electrodes 2a and 5, the electrowetting effect causes the fluid 7 to move so that it is adjacent to electrode 2a. When the voltage is removed, the droplet of fluid 7 should remain in this position as represented in FIG. 1(a). In this state, the droplet of fluid 7 is obscured by the black mask 8 from the viewer 9, and the viewer will see either the reflection from the electrode 2b in the case that the electrode 2b is reflective, or light from a backlight 10 in the case that the electrode 2b is transparent, i.e. this corresponds to the bright state of the display. If the voltage is now applied between electrodes 2b and 5 instead, the droplet of fluid 7 will now move so that it is adjacent to electrode 2b, and when the voltage is removed, it should remain in this position as represented in FIG. 1(b). In this state, the droplet of fluid 7 is now visible to the viewer 9, and since it is black, that pixel 21 will now appear dark, in either the case of a transmissive or a reflective display.
The problem with this very simple display is that a large portion of the surface area of the display (at least 50%) is covered with the black mask 8, and hence the brightness of the white state is very limited. An alternative scheme is described in US 2007/0127108 A1 (R. Hayes et al.; published Jun. 7, 2007), and illustrated in FIGS. 2(a) and 2(b). The structure of the device is very similar to that shown in FIGS. 1(a)-1(b), except that the upper substrate 1b does not have a hydrophobic surface, so that when no voltage is applied, the polar fluid 7 lies above a layer of the non-polar fluid 6 which wets the hydrophobic surface 4 on the lower substrate 1a. When a voltage is applied between the upper (5) and lower (2) electrodes, the polar fluid 7 now tries to wet the hydrophobic surface 4, causing the non-polar fluid 6 to break up into small droplets that occupy a much smaller area of the display substrate. If in this example the polar fluid 7 is transparent, and the non-polar fluid 6 is black (as illustrated in FIGS. 2(a)-2(b)), then in the voltage-off state, the pixel 21 will appear black as represented in FIG. 2(a), and in the voltage-on state the pixel 21 will appear almost completely transparent or completely reflective (depending on whether the rear electrode 2 is transparent or reflective) apart from the areas which are still occupied by the black, non-polar fluid 6, as represented in FIG. 2(b). This will correspond to the bright state whether the device is being operated in reflective or transmissive mode. It is apparent from the fact that the non-polar fluid 6 is made to “bunch-up” rather than translate sideways as in the previous mode, that this mode of operation offers a much better aperture ratio (as evidenced by the smaller area of black mask 8 in FIG. 2(b)), and therefore potentially a much higher brightness in the bright state. However, the aperture ratio is still limited by two factors. One is that the residual area occupied by the black fluid 6 is dependent on the voltage applied: the higher the voltage, the lower the residual area. However, because there will always be a maximum voltage that can be applied, the residual area will always be larger than the theoretical minimum. The other reason is that in practice it is necessary to use a pixel wall structure 24 in order to both separate the black fluid in one pixel from that in its neighbour, and in order to create a “seed” which will determine how the non-polar fluid layer breaks up into multiple smaller droplets. The effect of the pixel walls 24 is to further increase the unusable area of the pixel, reducing the aperture ratio. Current state-of-the-art aperture ratios are around 42% for this type of electrowetting display.
This last electrowetting display mode is currently being used by Philips spin-out company Liquavista to create monochrome and colour reflective display demonstrators capable of displaying video rate images. It has the distinct advantage over LCDs that it is polariser-free, and hence there is not the immediate 50% loss in brightness associated with absorbing one polarisation of the ambient light. However, as explained above, the current aperture ratios are of the order of 42%, leading to an overall white state reflectivity of just 37%, which is actually less than that normally available with LCDs. The reason for this is that although LCDs lose around 50% from the polariser, the aperture ratio for reflective LCDs is extremely good, usually at least 90-95%. This is not to be confused with the (rather lower) aperture ratios that can be achieved in transmissive LCDs. In this case the thin-film-transistors (TFTs) needed to drive each pixel of the display compete for space with the active area of each pixel, and therefore limit the aperture ratio. In a reflective LCD, however, the TFTs can be positioned underneath the pixel electrode, and the light never reaches them, hence the pixel electrode can occupy almost all of the available space dedicated to each pixel. It is only if the effective aperture ratios can be significantly improved that the white state reflectivity of electrowetting displays will begin to compete with and overtake that of reflective LCDs.
There are many ways in which the aperture ratio of electrowetting displays can be improved. For example WO2009/036272 A1 (J. Heikenfeld et al.; published Mar. 19, 2009) and US2008130087 A1 (A. Miyata et al., published Jun. 5, 2008) describe a way for improving the aperture ratio by moving one of the fluids into a vertical reservoir which reduces the area taken up by the fluid when not in “on display”. The authors of WO2009/036272 A1 claim aperture ratios of up to 95% should be possible. A second way of improving aperture ratio is to use a lower horizontal layer as a reservoir instead of a vertical channel, as described in U.S. Pat. No. 7,359,108 B2 (R. Hayes et al., issued Apr. 15, 2008). This arrangement has the advantage of bistability, however the time taken for the fluid to move from one layer to another can be quite large, resulting in poor switching speeds. A further, simpler method, which would potentially have an excellent aperture ratio is disclosed in US2009/0046082 A1 (J. Jacobson et al., published Feb. 19, 2009) and is simply to include two electrowetting fluid layers within the display, one on-top of the other, with electrodes and dielectric layers on both substrates. This is a bistable system where either the polar fluid or the non-polar fluid can be the upper layer, depending on which electrode has most recent had voltage applied to it, with reference to the polar fluid. It seems likely that this would require a relatively high switching voltage to go from one state to the other. These last two methods can also be applied only to reflective displays, whereas all of the previous examples were applicable to both transmissive and reflective displays.
The electrowetting geometries discussed so far are in the context of a monochrome or single colour display. In order to generate a colour display from any of these methods, they would require the addition of additive spatial colour filters (such as the traditional red, green, blue (RGB) system) to a monochrome display, which would further reduce the white state reflectivity by a factor of approximately 2 or 3, depending on whether a RGBW or RBG colour scheme is employed, respectively.
An alternative to creating a colour display by using additive spatial colour filters to a monochrome display (which is very wasteful in terms of brightness) is to make multi-layered cells using coloured (instead of black) dyes so that a subtractive technique can be used to create coloured pixels. This technique can be used with 3 or 4 electrowetting layers (e.g. cyan, yellow, magenta and an optional black layer) as disclosed in WO2003/071347 A1 (B. Feenstra et al., published Aug. 28, 2003) and U.S. Pat. No. 7,359,108 B2. The principle difficulty with this type of approach is that the individual layers must be positioned very close to each other (in the direction of the display normal), otherwise there is parallax between the various layers, and a blurred, colour separated or otherwise low-quality image is formed. Although significant advances in this field have been made recently using very thin, plastic substrates, these have generally been for non-pixelated colour changing elements, or used passive matrix addressing. It would therefore be timely to find a way in which to improve the effective aperture ratio of single-layer electrowetting devices.
WO2007/069179 A2 (S. Roosendaal et al., published Jun. 21, 2007) describes a method for improving the brightness of reflective displays by incorporating a wide-angle scatterer into the inactive portions of the display. The idea is that if the ambient light which strikes this surface is scattered out at a large angle, then some will not immediately escape the display due to the high to low refractive index change at the top surface of the display, i.e. it will be totally internally reflected. This would be a way of improving the brightness of the display, however the contrast ratio will be compromised because some of the light scattered from the in-active portions of the display will not be totally-internally-reflected and therefore exit the display directly.
WO 2008/122921 A1 (S. Roosendaal et al., published Oct. 16, 2008) describes an alternative method for improving the brightness of reflective displays by the addition of an external film on-top of the display. The additional film incorporates reflective or refractive structures intended to redirect the ambient light towards the active area of the pixel. However, because the redirecting structures are distant from the pixels (by the thickness of the top substrate of the display), such structures are unlikely to be effective over a large range of angles of incident light and/or viewing angle. In practice it is necessary to place the redirecting structures in very close proximity to the image-forming part of the display, in order to obtain good optical contrast.
U.S. Pat. No. 7,616,368 B2 (N. Hagood; issued Nov. 10, 2009) describes exactly just such an arrangement, where the redirecting structures are shaped like compound parabolic concentrator (CPC) or other light-concentrating structures, and placed in close proximity to the pixels of a microelectromechanical system (MEMS) in-plane-shutter style display. The purpose of the light-concentrating structures is as described above: to redirect the ambient light towards the active area of the pixel, in order to avoid light absorption by the other parts of the pixel which would normally be covered by black mask.